1. Field of the Invention
The present invention relates to a thin film semiconductor device such as a thin film transistor device, for use in a liquid crystal display device, an image sensor and the like, which is produced at a low temperature, and a method for producing such a thin film semiconductor device.
2. Description of the Related Art
In conventional field effect transistors including an active layer formed of polysilicon, micro-crystalline silicon or noncrystalline silicon, a great number of trapping levels and/or surface states are caused by defects in the active layer. The existence of such a great number of trapping levels and/or surface states lowers the mobility of electrons and holes and thus the threshold voltage fluctuates. In order to solve such problems, a so-called "hydrogen passivation method" has been used, by which the defects in the active layer are reduced by hydrogen, thereby lowering the number of defects in an area unit.
As a hydrogen passivation method, the following three methods have been proposed:
(1) Exposing the structure including an active layer to hydrogen plasma at a substrate temperature of 300.degree. C. to 400.degree. C., thereby introducing hydrogen into the active layer (hereinafter, referred to as the "hydrogen plasma method");
(2) Implanting hydrogen ions into the active layer and then annealing the structure including an active layer (hereinafter, referred to as the "hydrogen ion implantation method"); and
(3) Coating the structure including an active layer with silicon nitride including hydrogen (hereinafter, referred to as SiN:H) and then annealing the structure, thereby diffusing hydrogen from the silicon nitride layer into the active layer.
According to the hydrogen plasma method or the hydrogen ion implantation method, damage to the active layer cannot be avoided when the hydrogen is introduced or implanted into the active layer. Further, especially in the case where a planar transistor having a gate electrode formed of a silicon material is produced by the hydrogen plasma method, the speed of hydrogen passivation is low, and thus there is a problem in terms of throughput.
Under these circumstances, the method of diffusing hydrogen in the SiN:H layer into the active layer by heating is considered to be advantageous as a hydrogen passivation method. Generally in the case of a thin film transistor including a substrate formed of silicon, hydrogen passivation achieves great effects when a SiN:H having a sufficient compressive stress during the passivation anneal is used (G. P. Pollack et al., "Hydrogen Passivation of Polysilicon MOSFET's from a Plasma Nitride Source", IEEE Electron Device Lett. vol. EDL-5, No. 11, November, 1984). By contrast, in the case of a transistor having an insulating substrate formed of a material such as quartz, hydrogen passivation does not realize sufficient improvement of transistor characteristics even by use of the above-mentioned SiN:H.